Fabrication of semiconductors requires monitoring and controlling temperature during process steps. Sensitivity and accuracy over a wide temperature range (e.g. room temperature to approximately 1000.degree. C.) is important. Speed with which real time corrective response is provided is additionally required. Thermocouples, optical pyrometers and fluorescent techniques are well known and their limitations are well documented. Thermo-couples are inaccurate in non-contact mode particularly in vacuum environment. Pyrometry methods rely on knowledge of surface emissivity which is generally varying during the semiconductor process applications. It may be impossible to use pyrometry at low temperatures when semiconductor substrates are transparent to emission from a heater. Fluorescence is not generally suitable for measurements above 450.degree. C., particularly in those combinations employing combustible binders.
It is also known in contact type measurements to determine temperature using a material which exhibits a light absorption characteristic which is a function of temperature. U.S. Pat. No. 4,669,872 discloses the use of GaAs as a material having an absorption characteristic which is useful in temperature determining applications. That patent discloses an attempt to calculate the wavelength at the GaAs absorption band edge and uses the known correlation between the band edge wavelength to substrate temperature. U.S. Pat. No. 4,790,669 also uses GaAs as a temperature determining element and attempts to determine a reference wavelength value by measuring the wavelengths at 1/2 peak intensity and using an algorithm to establish the temperature. It is also known in molecular beam epitaxy of layers onto GaAs substrates to use the known band edge absorption characteristic of GaAs in an attempt to determine the temperature of the substrate during the layering process. See Hellman, et al., Infra-Red Transmission Spectroscopy of GaAs During Molecular Beam Epitaxy, Journal of Crystal Growth, v. 81 (1987) pp. 38-42. This prior art technique as depicted by the Hellman article is not amenable to commercial application as stated by Hellman because of its slowness, complexity and because its absolute accuracy was quite poor (.+-.10.degree. C.). The Hellman technique is also unfavorable because it would be difficult to apply to doped substrates, requires accurate knowledge of the precise thickness of the substrates and is affected by presence of thin films on the substrate and windows which can interfere and mask the true temperature measurement.